The cellular processes that are regulated by nitric oxide (NO) are central to many aspects of biology and disease, particularly in the cardiovascular system and the central and peripheral nervous systems. Despite the widely recognized importance of NO, little is known about the mechanism of regulation of the NO receptor, the soluble guanylyl cyclase (sGC). sGC is a heme-containing heterodimer that catalyzes the formation of cGMP from the substrate GTP. Upon binding of NO to the heme, the sGC is activated several hundred-fold and it is thought that the NO-cGMP signaling cascade is a key player in regulation of blood pressure, synaptic plasticity and inhibition of platelet aggregation. The proposed studies seek to understand the structural basis of mechanisms of regulation of the sGC and to identify modulators of its activity: 1) Our recent work suggests that sGC contains an allosteric regulatory site. We seek to identify residues that dictate the structure and nature of the proposed allosteric site. Guided by homology with adenylyl cyclases, we conducted a mutational analysis that identifies specific regions of interactions between the two subunits that seem to mediate allosteric activation. We shall characterize these mutants by biochemical studies integrated with molecular modeling. 2) Recent work from our laboratory and others, suggests that there is an endogenous regulator for the sGC. We have discovered but not yet identified a novel endogenous activator. We will identify this activator and screen for others. 3) The wild-type and mutant sGC will be used to test a newly generated chemical library derived from a synthetic activator, YC-1. Active compounds and sGC will be used as templates for molecular modeling. This modeling combined with structure-activity relationship studies will be used to generate detailed and testable hypotheses regarding the physiological modulation of the NO-cGMP signaling pathway. Understanding the mechanisms of regulation of sGC and identifying regulatory molecules will be key to uncovering the molecular basis of some types of hypertension, atherosclerosis and erectile dysfunction. [unreadable] [unreadable]